In the past, almost all devices for converting rotary motion to rectilinear motion have been plagued by side thrust (i.e., forces acting at right angles to the direction of linear movement). Side thrust is disadvantageous because it can cause considerable wear in the reciprocating parts of such devices.
U.S. Pat. Nos. 947,233; 2,775,128 and 4,078,439 relate to devices which attempt to overcome side thrust by employing a slide connected to a crank or eccentric through a rotary bearing mounted in a relatively large revolving disc. Because the revolving discs have to be mounted in bearing races, these prior devices would be expensive and limited to circular configurations with diameters less than the width of the slide. Moreover, it has been found that the large bearing surfaces provided by the rotating discs of such devices offer no real advantage since the stress on the bearing is always acting essentially at a point, and, therefore, the bearing does not distribute such stress to any substantial degree.